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+- Repo: eslint/markdown
+- Start Date: 2025-08-19
+- RFC PR: https://github.com/eslint/rfcs/pull/138
+- Authors: 루밀LuMir(lumirlumir)
+
+# Support GFM alert syntax parsing
+
+## Summary
+
+This RFC proposes adding support for GFM alert syntax parsing to the [`@eslint/markdown`](https://github.com/eslint/markdown) package by implementing `micromark-extension-gfm-alert` and `mdast-util-gfm-alert` custom plugin logic, and also includes the formal specification based on the [CommonMark spec](https://spec.commonmark.org/) and the [GitHub Flavored Markdown spec](https://github.github.com/gfm/).
+
+## Motivation
+
+The motivation for this RFC comes from [eslint/markdown#294](https://github.com/eslint/markdown/issues/294).
+
+Currently, GitHub supports a special syntax for alerts that isn't mentioned in the [GitHub Flavored Markdown spec](https://github.github.com/gfm/) and isn't parsed by the existing [`micromark-extension-gfm`](https://github.com/micromark/micromark-extension-gfm?tab=readme-ov-file#when-to-use-this) and [`mdast-util-gfm`](https://github.com/syntax-tree/mdast-util-gfm?tab=readme-ov-file#when-to-use-this) packages, which are used internally by `@eslint/markdown` to parse Markdown content and create AST nodes.
+
+Inconsistencies between GitHub's GFM alert syntax and existing Markdown parsers can cause false positives and false negatives in some cases, especially when writing rules that detect error-prone syntax and when the syntax overlaps with GitHub's alert syntax.
+
+This RFC aims to address that gap by introducing the necessary parsing logic and formal specification for GFM alert syntax.
+
+GFM alert syntax is a way to create attention-grabbing blocks in Markdown content. It uses a specific syntax to denote different types of alerts, such as notes, tips, warnings, and more. The basic structure of a GFM alert is as follows:
+
+```md
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+> [!TIP]
+> Helpful advice for doing things better or more easily.
+
+> [!IMPORTANT]
+> Key information users need to know to achieve their goal.
+
+> [!WARNING]
+> Urgent info that needs immediate user attention to avoid problems.
+
+> [!CAUTION]
+> Advises about risks or negative outcomes of certain actions.
+```
+
+Which is being rendered on GitHub as:
+
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+> [!TIP]
+> Helpful advice for doing things better or more easily.
+
+> [!IMPORTANT]
+> Key information users need to know to achieve their goal.
+
+> [!WARNING]
+> Urgent info that needs immediate user attention to avoid problems.
+
+> [!CAUTION]
+> Advises about risks or negative outcomes of certain actions.
+
+
+## Detailed Design
+
+### Terms
+
+Before explaining the design, it's important to define some key terms:
+
+- ***closing angle bracket***: The `>` (U+003E) character that denotes a block quote in Markdown.
+- ***opening square bracket***: The `[` (U+005B) character that starts the alert syntax.
+- ***closing square bracket***: The `]` (U+005D) character that ends the alert syntax.
+- ***exclamation mark***: The `!` (U+0021) character is used as a prefix to denote the alert type.
+- ***space***: The ` ` (U+0020) character that separates the block quote marker from the alert syntax.
+- ***tab***: The `\t` (U+0009) character that represents a tab.
+- ***line ending***: According to the [CommonMark spec](https://spec.commonmark.org/0.31.2/#line-ending), A line ending is a line feed (U+000A), a carriage return (U+000D) not followed by a line feed, or a carriage return and a following line feed.
+- ***label***: The text that appears inside the ***opening square bracket*** and ***closing square bracket*** and is prefixed by an ***exclamation mark***. It determines the style and semantics of the alert.
+
+### GFM alert syntax in plugins and configs
+
+This suggestion requires adding an option to `languageOptions` in the `@eslint/markdown` to enable parsing of GFM alert syntax.
+
+`micromark-extension-gfm-alert` and `mdast-util-gfm-alert` complement each other and are intended to be used together to provide full support for the GFM alert syntax.
+
+To enable parsing of GFM alert syntax, set the `languageOptions.alert` option to `true`; set it to `false` to disable it. Only the boolean values `true` and `false` are accepted.
+
+Adding the `alert` option to the [Language Options](https://github.com/eslint/markdown?tab=readme-ov-file#language-options) and set it to `false` by default will not affect existing functionality in `@eslint/markdown@7` (version 7.x.x).
+
+If desired, this option could be enabled by default in `@eslint/markdown@8` (version 8.x.x).
+
+```diff
+// eslint.config.js
+import { defineConfig } from "eslint/config";
+import markdown from "@eslint/markdown";
+
+export default defineConfig([
+    {
+        files: ["**/*.md"],
+        plugins: {
+            markdown
+        },
+        language: "markdown/gfm",
+        languageOptions: {
+            frontmatter: "yaml", // Or pass `"toml"` or `"json"` to enable TOML or JSON front matter parsing.
++           alert: true // Enable GFM alert syntax parsing (default: `false`)
+        },
+        rules: {
+            "markdown/no-html": "error"
+        }
+    }
+]);
+```
+
+### Where would the `micromark-extension-gfm-alert` and `mdast-util-gfm-alert` packages live?
+
+#### `micromark-extension-gfm-alert`
+
+- `micromark-extension-gfm-alert` source code logic would be implemented in the `src/extensions/micromark-extension-gfm-alert.js` file.
+- `micromark-extension-gfm-alert` test files would be implemented in the `tests/extensions/micromark-extension-gfm-alert.test.js` file.
+
+#### `mdast-util-gfm-alert`
+
+- `mdast-util-gfm-alert` source code logic would be implemented in the `src/extensions/mdast-util-gfm-alert.js` file.
+- `mdast-util-gfm-alert` test files would be implemented in the `tests/extensions/mdast-util-gfm-alert.test.js` file.
+
+### Type Interface
+
+Since GFM alert syntax is part of blockquote syntax from Markdown, `Alert` interface extends [`Blockquote`](https://github.com/syntax-tree/mdast?tab=readme-ov-file#blockquote) node type of Mdast.
+
+```ts
+import type { Blockquote } from "mdast";
+
+interface Alert extends Blockquote {
+    /**
+     * Node type of mdast GFM alert.
+     */
+    type: "alert",
+
+    /**
+     * The label of the alert. It determines the style and semantics of the alert.
+     * 
+     * Its value should remain in its parsed form and should not be normalized.
+     *
+     * You can normalize its value by converting it to lowercase.
+     * The normalized value should be one of the following: `"note"`, `"tip"`, `"important"`, `"warning"`, `"caution"`
+     */
+    label: string,
+}
+```
+
+### Valid Syntax
+
+Here are some examples of valid GFM alert syntax:
+
+#### All of the following are valid ***label***s: `NOTE`, `TIP`, `IMPORTANT`, `WARNING` and `CAUTION`
+
+```md
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [!TIP]
+> Helpful advice for doing things better or more easily.
+```
+
+> [!TIP]
+> Helpful advice for doing things better or more easily.
+
+---
+
+```md
+> [!IMPORTANT]
+> Key information users need to know to achieve their goal.
+```
+
+> [!IMPORTANT]
+> Key information users need to know to achieve their goal.
+
+---
+
+```md
+> [!WARNING]
+> Urgent info that needs immediate user attention to avoid problems.
+```
+
+> [!WARNING]
+> Urgent info that needs immediate user attention to avoid problems.
+
+---
+
+```md
+> [!CAUTION]
+> Advises about risks or negative outcomes of certain actions.
+```
+
+> [!CAUTION]
+> Advises about risks or negative outcomes of certain actions.
+
+#### All of the following are case-insensitive ***label***s: `NOTE`, `TIP`, `IMPORTANT`, `WARNING` and `CAUTION`
+
+The example below shows only `NOTE`, but it applies to all labels such as `TIP`, `IMPORTANT`, `WARNING`, and `CAUTION`.
+
+```md
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [!note]
+> Useful information that users should know, even when skimming content.
+```
+
+> [!note]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [!Note]
+> Useful information that users should know, even when skimming content.
+```
+
+> [!Note]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [!NoTe]
+> Useful information that users should know, even when skimming content.
+```
+
+> [!NoTe]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [!nOtE]
+> Useful information that users should know, even when skimming content.
+```
+
+> [!nOtE]
+> Useful information that users should know, even when skimming content.
+
+#### Upto 4 indented ***space*** is allowed before the ***opening square bracket***
+
+The example below shows only `NOTE`, but it applies to all labels such as `TIP`, `IMPORTANT`, `WARNING`, and `CAUTION`.
+
+```md
+>[!NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+>[!NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+>  [!NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+>  [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+>   [!NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+>   [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+>    [!NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+>    [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+#### Upto 3 indented ***space*** is allowed before the ***closing angle bracket***
+
+```md
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+ > [!NOTE]
+ > Useful information that users should know, even when skimming content.
+```
+
+ > [!NOTE]
+ > Useful information that users should know, even when skimming content.
+
+---
+
+```md
+  > [!NOTE]
+  > Useful information that users should know, even when skimming content.
+```
+
+  > [!NOTE]
+  > Useful information that users should know, even when skimming content.
+
+---
+
+```md
+   > [!NOTE]
+   > Useful information that users should know, even when skimming content.
+```
+
+   > [!NOTE]
+   > Useful information that users should know, even when skimming content.
+
+### Invalid Syntax
+
+Here are some examples of invalid GFM alert syntax:
+
+#### ***exclamation mark*** should not be surrounded by any ***space***s or ***tab***s
+
+```md
+> [ !NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> [ !NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [    !NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> [    !NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [! NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> [! NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [!    NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> [!    NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [ ! NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> [ ! NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [    !    NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> [    !    NOTE]
+> Useful information that users should know, even when skimming content.
+
+#### ***Label***s should not end with ***space***s or ***tab***s
+
+```md
+> [!NOTE ]
+> Useful information that users should know, even when skimming content.
+```
+
+> [!NOTE ]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [!NOTE    ]
+> Useful information that users should know, even when skimming content.
+```
+
+> [!NOTE    ]
+> Useful information that users should know, even when skimming content.
+
+#### Only ***space***s and ***tab***s are allowed before the ***opening square bracket***
+
+```md
+> hi[!NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> hi[!NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> hi [!NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> hi [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+#### Only ***space***s, ***tab***s, and ***line ending***s are allowed after the ***closing square bracket***
+
+```md
+> [!NOTE]hi
+> Useful information that users should know, even when skimming content.
+```
+
+> [!NOTE]hi
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [!NOTE] hi
+> Useful information that users should know, even when skimming content.
+```
+
+> [!NOTE] hi
+> Useful information that users should know, even when skimming content.
+
+#### GFM alert syntax without content isn't working
+
+```md
+> [!NOTE]
+```
+
+> [!NOTE]
+
+---
+
+```md
+> [!NOTE]
+>
+```
+
+> [!NOTE]
+>
+
+#### Multi-line GFM alert syntax isn't working
+
+```md
+> [
+> !NOTE]
+> Useful information that users should know, even when skimming content.
+```
+
+> [
+> !NOTE]
+> Useful information that users should know, even when skimming content.
+
+---
+
+```md
+> [!NOTE
+> ]
+> Useful information that users should know, even when skimming content.
+```
+
+> [!NOTE
+> ]
+> Useful information that users should know, even when skimming content.
+
+#### Nested GFM alert syntax isn't working
+
+```md
+> > [!NOTE]
+> > Useful information that users should know, even when skimming content.
+```
+
+> > [!NOTE]
+> > Useful information that users should know, even when skimming content.
+
+---
+
+```md
+- > [!NOTE]
+- > Useful information that users should know, even when skimming content.
+```
+
+- > [!NOTE]
+- > Useful information that users should know, even when skimming content.
+
+#### GFM alert syntax should not be enclosed in HTML opening or closing tags
+
+```md
+<div>
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+</div>
+```
+
+<div>
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+</div>
+
+---
+
+```md
+<div>
+
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+</div>
+```
+
+<div>
+
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+</div>
+
+---
+
+```md
+<details>
+<summary>Details</summary>
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+</details>
+```
+
+<details>
+<summary>Details</summary>
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+</details>
+
+---
+
+```md
+<details>
+<summary>Details</summary>
+
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+</details>
+```
+
+<details>
+<summary>Details</summary>
+
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+
+</details>
+
+## Documentation
+
+We need to update the [`README.md`](https://github.com/eslint/markdown?tab=readme-ov-file#eslint-markdown-language-plugin) for `@eslint/markdown` to include details about the new GFM alert syntax parsing feature. In particular, we should document the new `alert` option under the [Language Options](https://github.com/eslint/markdown?tab=readme-ov-file#language-options) section.  
+
+## Drawbacks
+
+1. This is not part of the official CommonMark or GitHub Flavored Markdown specifications.
+2. It may not be widely adopted outside of GitHub, which could lead to inconsistencies in Markdown parsing across different platforms.
+
+## Backwards Compatibility Analysis
+
+This proposal is fully backward-compatible, as it only adds new options and does not remove or modify existing features.
+
+No changes are required from end users.
+
+## Alternatives
+
+I cannot find any significant alternatives to this proposal. but I am open to suggestions.
+
+### Pre-existing Implementations
+
+There are existing implementations of GFM alert syntax parsing in other Markdown and HTML parsers:
+
+- `rehype-github-alert`: https://github.com/rehypejs/rehype-github/tree/main/packages/alert#rehype-github-alert
+- `markdown-it-github-alert`: https://github.com/antfu/markdown-it-github-alerts
+
+However, these plugins cannot be used in the `eslint/markdown` package, because `rehype-github-alert` is a plugin for `rehype` (which uses [HAST](https://github.com/syntax-tree/hast#readme) AST) and `markdown-it-github-alert` is a plugin for `markdown-it` (which uses its own Markdown AST), while `eslint/markdown` parses Markdown using `micromark` and `mdast` (which uses [MDAST](https://github.com/syntax-tree/mdast#readme) AST).
+
+## Open Questions
+
+- Question 1. Will the `micromark-extension-gfm-alert` and `mdast-util-gfm-alert` plugins be published as separate packages?
+- Answer 1. According to the [comment](https://github.com/eslint/rfcs/pull/138#discussion_r2288501879), it is not preferred to have additional packages to maintain for this.
+
+## Help Needed
+
+I've been reviewing the `micromark-extension` and `mdast-util` guides and implementations, but I'm not sure how best to implement the `micromark-extension-gfm-alert` and `mdast-util-gfm-alert` plugins.
+
+I'd appreciate any guidance while I work on this, or help from anyone willing to take it on.
+
+## Frequently Asked Questions
+
+N/A
+
+## Related Discussions
+
+### The reasons why GitHub alert syntax is not included in Mdast
+
+- https://github.com/syntax-tree/mdast-util-gfm/issues/2
+- https://github.com/orgs/syntax-tree/discussions/144
+
+### `micromark-extension` references
+
+- Creating a micromark extension: https://github.com/micromark/micromark?tab=readme-ov-file#creating-a-micromark-extension
+
+### `mdast-util` references
+    
+- `mdast-util-math`: https://github.com/syntax-tree/mdast-util-math
+- `mdast-util-gfm-task-list-item`: https://github.com/syntax-tree/mdast-util-gfm-task-list-item
\ No newline at end of file